Waste Wood for Fuel
Today's biomass power plants primarily use residues from the wood-products industries, with 64% of the energy's being produced from wood and wood wastes. This compares with energy productions from municipal solid waste of 24%, agricultural waste of 5%, and landfill gases of 5%.
Waste-wood processing requirements depend on the source of wood waste and requirements of the wood-combusting facility. Wood commingled with other construction and demolition debris typically requires removal of nonwood materials and size reduction into chips prior to use as fuel. Fuel specifications for wood-fired power plants usually include minimum and maximum particle size, amount of fines, maximum moisture content, and quantity of contaminants.
The market potential for wood waste used as fuel is favorable where it is an indigenous, abundant, renewable-energy source and its low sulfur content has air emission benefits. Wood waste cofired with coal in utility boilers could result in significant reductions in acid gas emissions. However, the low price of fossil fuels makes wood fuel less cost competitive unless disposal charges (tipping fees) are paid to processors and users of wood waste.
Green wastes are generated by tree and landscape maintenance services and include: mixed wood, logs, and tops and brush; clearing and grubbing waste; leaves and grass clippings; and chips and whole stumps. Some green wastes may be unacceptable for charcoal because their high ash and alkali contents could cause slagging and fouling problems in boilers co-firing charcoal with coal.
Moisture affects the energy content of burning wood. Wood recently harvested from land clearing has a moisture content from 40% to 50% and a heating value of about 5500 Btu/lb. Wood residue from kilndried material has 5% to10% moisture and approximately 7500 Btu/lb. Wood separated from the waste stream typically has a moisture content of 12% to 15% and 6000 to 7500 Btu/lb. In comparison, sub-bituminous coal, which is used mostly for power generation, usually contains 20 to 30% moisture and has a heat content from 8,000 to 12,000 Btu/lb, as received.
Generally creosote--or penta-treated materials--treated wood removed from service has energy value and therefore could be considered an energy resource. Some states have co-generation or incinerator facilities that are properly permitted to burn treated as well as untreated wood waste.
New York State has extensively evaluated use of urban wood as a source of fuel for power production. Its work generally indicated that, with the exception of chromated-copper-arsenate (CCA) pressure-treated wood, the exposure risks from combusting most treated wood are slight or insignificant and meet state guidelines. CCA-treated wood, however, should not be used as a fuel for direct combustion.
The cost of electricity from wood-fired power plants on the U.S. mainland ranges from U.S.$0.065 to more than $0.09/kWh. With energy conversion rates of 14,000 to 18,000 Btu/kWh, these plants are 18 to 24% efficient. They therefore are competitive only when they can receive feedstock at very low prices.